System and method for focal length stabilization using active temperature control

ABSTRACT

An optical metrological system having a heat-generating light source coaxially mounted near a heat-sensitive lens. The system uses a temperature sensor to monitor the lens temperature and a heating element to heat the lens such that the lens operating temperature is greater than a maximum operating temperature of the light source in order to stabilize the focal length of the lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not applicable.

BACKGROUND AND SUMMARY

This invention is related to metrological methods and systems, and moreparticularly, to methods and systems for focal length stabilization ofmetrological systems using active temperature control.

Automated metrology systems are used for the optical inspection of anobject. Such inspections are performed in order to obtain precisedimensional and other measurements of the object. The object is placedon a stage (with precision movements for X-Y measurements) and the imageof the object undergoes computerized image analysis. The Z-axis is alsomeasurable using the auto-focus routine of the software resulting inheight measurements of the object. A precise three-dimensionalreproduction of the object is obtained using the measurements. Theoptical assemblies used in such systems are composed of a main imagingpath, a calibration system and a surface illumination system forillumination of the object to be inspected.

The main imaging system of an optical system used for metrology, asdescribed, for example, in U.S. Pat. No. 6,292,306, is comprised of afront lens, and either a fixed or zoom system behind, with a camera inthe focal plane of the system. The system is designed to have collimatedspace between the front lens and the zoom or fixed lens portion of theimaging system. Z-axis measurements are taken with auto-focus, acomputerized image analysis, to find the best focus of the system.Because of the collimated space behind the front lens, the front focusof the system is ultimately the front focus of the front lens. Anyenvironmental changes to the front lens that may cause the physicalfront focus to change will add error to the Z-axis measurement of theobject.

The calibration system, as described, for example, in U.S. Pat. No.5,389,774, allows a user to perform calibration and return to apreviously saved magnification. This is done by saving a reticle imageat a selected magnification, calling up that image when thatmagnification is desired again, and waiting for the zoom lens to adjustuntil the present reticle image matches the saved reticle image.

The surface illumination system uses a variety of techniques toilluminate the object to be measured in order to enhance the precisionof the measurements. One technique uses an LED ring surface illuminator,as described, for example, in U.S. Pat. No. 5,690,417, that allows forcontours, ledges, edges, and other generalized surface height variationsto be imaged. In such a system, the light source may surround the frontlens of the main imaging system, creating a large amount of heatadjacent to the lens. When heated, the properties of the glass change,thus causing a change in front focal length directly affecting the frontworking distance/front focus (Z-axis measurement/height) of the system.The problem this creates is inaccurate Z-axis position measurements ofthe object to be measured. The Z-axis position measurement of the objectwill change once the light source is turned on, and will continue tochange over time, as the light source heats up, until the light sourcereaches its maximum operating temperature. Any time the light is turnedoff and the temperature of the optics is allowed to change, the Z-axisposition measurement of the object will also change. The fluctuation oflens focal length with temperature also results in repeatability errorsas confirmed by repeated measurements of the same object over a periodof time. Typically, Z-axis position measurements may fluctuate by 10–20microns due to a temperature change, depending on the optical system andthe amount of heat generated by the light source. An advantage ofembodiments is the reduction of Z-axis position measurement variationsin an object measured by a metrology system by reducing the temperaturefluctuation in a heat-sensitive lens.

Embodiments stabilize the lens temperature of an optical system having aheat-sensitive lens in proximity to a heat generating device such as alight source. Embodiments also insulate the lens from environmentaltemperature variations that can affect Z-axis position measurements.Embodiments can be used in any application in which a stable lenstemperature can optimize system performance. Embodiments provide anadvantage in that they enable accurate Z-axis position measurements ofobjects and ensure that such measurements do not vary over time.Maintaining the lens at a constant temperature can also protect the lensfrom damage such as de-cementing which is caused by sudden and frequentlens temperature variations.

An optical system with focal length stabilization in accordance withembodiments includes a housing supporting a heat-sensitive lens withinthe housing, a light source secured to the housing, a heating elementconnected to the housing to heat the lens, at least one temperaturesensor connected to the housing, and a controller in electricalcommunication with the at least one temperature sensor and the heatingelement such that the controller monitors a temperature of the lens andadjusts a current in the heating element to maintain the temperature ofthe lens within a pre-selected range.

A method for stabilizing focal length in a heat-sensitive lens supportedin a housing having a light source secured thereto in accordance withembodiments includes the steps of monitoring a lens temperature using atleast one temperature sensor connected to the housing, and maintainingthe lens temperature within a pre-selected range by controlling aheating element attached to the housing

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of an optical system having a lightsource coaxially mounted on one end of its objective lens system andusing a temperature sensor and heating element for active focal lengthstabilization in accordance with embodiments.

FIG. 2 is a bottom view of an optical system having a light sourcecoaxially mounted on one end of its objective lens system in accordancewith embodiments.

FIG. 3 is a schematic block diagram of a temperature control system forstabilizing focal length in an objective lens system in accordance withembodiments.

FIG. 4 is a side elevation view of an optical system having a lightsource coaxially mounted on one end of its objective lens system andusing multiple temperature sensors, a heating element, and an indicatorfor active focal length stabilization in accordance with embodiments.

FIG. 5 is a schematic flow diagram of a method for stabilizing focallength in a heat-sensitive lens in accordance with embodiments.

DESCRIPTION

A system 10 and method for stabilizing focal lengths in a heat sensitivelens in accordance with embodiments is illustrated in FIGS. 1-5. Thesystem 10 includes a cylindrical lens housing 12 containing aconventional objective lens system 30, a lamp supporting housing 14, atemperature sensor 16, a heating element 18, a controller 20, and alight source 22, although the system 10 can comprise other numbers andtypes of components in other configurations. The method describesstrategically heating the housing and maintaining the temperature of aheat-sensitive lens within a pre-selected range in order to stabilizethe focal length of the lens. Embodiments provide a number of advantagesincluding the reduction of inaccurate measurements and repeatabilityerrors caused by heat-induced variations in lens focal length.

Referring to FIGS. 1 and 2, the system 10 includes a cylindrical lenshousing 12 containing a conventional objective lens system 30, adisc-shaped lamp supporting housing 14, a temperature sensor 16, aheating element 18, and a controller 20 that adjusts current to theheating element to increase and decrease temperature measured at thetemperature sensor. In the exemplary embodiment shown in FIGS. 1 and 2,an annular, generally disc-shaped lamp supporting housing 14 is securedto and surrounds the lower end of the lens housing 12. A light source 22is mounted in the lamp supporting housing 14. Preferably, the lightsource comprises a plurality of lamps L, although any suitable type oflight source can be used. The lamps L are secured or mounted at theirinner ends in the lamp supporting housing 14, and the lamps L project attheir outer, light emitting ends downwardly from the housing 14 in thedirection of an object 24, which object 24 rests on a work table 26. Asshown more clearly in FIG. 2, in embodiments the lamps L are mounted inthe lamp supporting housing 14 in five circular arrays or rings disposedcoaxially of the axial centerline of the housings 12 and 14. The lamps Lare located proximate to and typically surround the lens 30 and cancreate large amounts of heat in the area around the lens 30. The lens30, which is corrected for color aberrations, is sensitive to heat. Whenheated, the focal length of the lens 30 changes with temperature,directly affecting the front working distance and front focus of thesystem, which can result in distortion of the perceived Z-axis distance.

Referring to FIG. 3, a temperature control system preferably includesthe controller 20, heating element 18, and temperature sensor 16. Thecontroller 20 includes a memory 80, a processor 82, an input/output unit84, and an indicator 86, which are connected together by a bus 88 orother link although other suitable types and numbers of components inother configurations and other types of processing systems can be usedfor the controller. In alternative embodiments, all of the componentsare placed on a single microchip or semiconductor device. The memory 80can store instructions and data for performing one or more aspects ofembodiments, including the methods described with references to FIGS.1-5, although some or all of these instructions and data can be storedelsewhere. A variety of different types of memory storage techniques,such as a random access memory (RAM), a read only memory (ROM), flashROM, EEPROM, and the like, and even hard disk drives, can be used by thememory 80 to store the instructions and data.

Referring to FIGS. 1, 2 and 3, the heating element 18 comprises heattape, preferably of the OMEGA® KAPTON type of insulated flexible heaters(catalog no. khlv 0502/5-p), although other suitable types of heatingelements can be used. The heating element 18 is preferably wrappedaround the circumference of the cylindrical lens housing 12 as closelyas possible to the lens 30, although other locations and techniques forattaching the heating element 18 can be used. The temperature sensor 16comprises a thermocouple, although other types of temperature sensorscan be used, and is preferably mounted on the cylindrical lens housing12 as closely as possible to the lens 30, although other locations forattaching the temperature sensor 16 can be used. The temperature sensor16 is preferably attached with adhesive, though other attachmenttechniques can be used.

Referring again to FIG. 3, the controller 20 in embodiments isoperatively connected to the heating element 18 and temperature sensor16 by wire, although other techniques for connecting the devices may beused, such as wireless communications techniques. The temperature sensor16 preferably transmits a temperature measured proximately to lens 30 orsignal representative thereof to the controller 20 which, in turn,increases or decreases current to the heating element 18 as required tomaintain the temperature of the lens 30 within a target temperaturerange in accordance with methods disclosed herein.

Referring to FIG. 4, other embodiments for stabilizing focal lengths ina heat sensitive lens will now be described. The system 50 ofembodiments includes a cylindrical lens housing 52 containing aconventional objective lens system 54, a disc-shaped lamp supportinghousing 56, a light source 58 in the lamp supporting housing 56,temperature sensors 60, 62, and 64, a heating element 66, an indicator86, and a controller 70. In such embodiments, the controller 70preferably calculates a weighted average temperature of the lens system54 based on inputs from temperature sensors 60, 62, and 64, althoughother calculating methods can be used. The controller 70 of embodimentsthen adjusts current to the heating element 66 as described herein untilthe temperature measured at the lens system 54 by temperature sensors60, 62, and 64 falls within the target temperature range. The entiretarget temperature range is preferably greater than the maximumoperating temperature of the light source 58 to minimize temperaturefluctuations, and hence minimize focal length-drift in the lens system54. The indicator 86 is preferably illuminated when the temperaturemeasured at the lens system 54 is within the target temperature range inorder to advise a user that the focal length of the lens system 54 hasachieved the desired stability.

Referring to FIGS. 1, 3, and 5, these FIGS. illustrate an example of amethod for stabilizing focal length in a heat-sensitive lens by heatingthe lens to a temperature that is greater than the maximum operatingtemperature of a light source surrounding the lens in accordance withembodiments. The method preferably comprises monitoring the temperatureof the lens 30 using the temperature sensor 16 and maintaining thetemperature of the lens 30 within a pre-selected range that is greaterthan the maximum operating temperature of a light source 22 surroundingthe lens by controlling a heating element 18 attached to a cylindricalhousing 12 surrounding the lens 30. The pre-selected range inembodiments comprises a minimum and maximum temperature that can bestored in the internal memory of the controller 20, entered into thecontroller 20 by a user, or provided to the controller by an externalsensor such as a temperature sensor. The operating temperature range forthe lens 30 will preferably be established based on the accuracy desiredfor the Z-axis measurements. Heating the lens 30 to the targettemperature range and maintaining the lens temperature within that rangewill ensure the focus will remain constant whether the light source 22has recently been turned on, remains on for a long period, or is turnedoff, so long as the entire range is set to be greater than the maximumoperating temperature of the light source 22.

In FIG. 5, at block 100, the temperature sensor 16 of embodimentstransmits a temperature measured proximately to lens 30 or signalrepresentative thereof to the controller 20. At block 110, thecontroller 20 preferably compares the temperature or signal receivedfrom the sensor 16 to a pre-determined minimum temperature that isstored in the internal memory 80 of the controller 20. If thetemperature received from the sensor 16 is less than the range minimum,then at block 120 the controller 20 of embodiments increases current tothe heating element 18, after which the system returns to block 100. Ifthe temperature received from the sensor 16 is greater than the rangeminimum, then at block 130 the controller 20 of embodiments compares thetemperature received from sensor 16 to a pre-determined maximumtemperature that is also stored in the internal memory 80 of thecontroller 20. If the temperature received from the sensor 16 is greaterthan the range maximum, then at block 140 the controller 20 ofembodiments decreases current to the heating element 18, after which thesystem returns to block 100 to perform another temperature measurement.If the temperature received from the sensor 16 is less than the rangemaximum, then at block 150 the controller 20 of embodiments maintainsthe current to the heating element 18 at its present level, and thecontroller 20 returns to block 100 to receive another temperaturemeasurement.

While particular embodiments have been described, alternatives,modifications, variations, improvements, and substantial equivalentsthat are or may be presently unforeseen may arise to applicants orothers skilled in the art. Accordingly, the appended claims as filed,and as they may be amended, are intended to embrace all suchalternatives, modifications, variations, improvements, and substantialequivalents. Further, the recited order of processing elements orsequences, or the use of numbers, letters, or other designationstherefore, is not intended to limit the claimed processes to any orderexcept as may be specified in the claims.

1. An optical system having focal length stabilization, the systemcomprising: a housing supporting a heat-sensitive lens within thehousing; a light source secured to the housing; at least one heatingelement connected to the housing to heat the lens; at least onetemperature sensor connected to the housing; and a controller inelectrical communication with the at least one temperature sensor andthe heating element such that the controller monitors a lens temperatureof the lens and adjusts a current in the heating element to maintain thelens temperature within a pre-selected range.
 2. The system of claim 1wherein the lens temperature is greater than a reference temperature. 3.The system of claim 1 wherein the lens temperature is greater than amaximum operating temperature of the light source.
 4. The system ofclaim 1 wherein the controller maintains the lens temperature at asubstantially fixed value that is greater than a maximum operatingtemperature of the light source, whereby changes in a front focal lengthof the lens caused by temperature variation are minimized.
 5. The systemof claim 4 further comprising an indicator that is actuated when thelens temperature is greater than the maximum operating temperature ofthe light source.
 6. The system of claim 1 wherein the light source issecured to and surrounds a lower end of the housing.
 7. The system ofclaim 6 wherein the light source further comprises an annular lamphousing containing a plurality of radially spaced circular arrays oflamps.
 8. The system of claim 7 wherein the lamps are light emittingdiodes.
 9. The system of claim 1 wherein the temperature sensor is athermocouple.
 10. The system of claim 1 wherein the pre-selected rangeis programmed into the controller.
 11. A heat-sensitive lens focallength stabilization method wherein the lens is supported in a housinghaving a light source secured thereto, the method comprising: monitoringa lens temperature of the lens using at least one temperature sensorconnected to the housing; providing a pre-selected lens temperaturerange; and maintaining the lens temperature within the pre-selectedrange by controlling a heating element attached to the housing.
 12. Themethod of claim 11 wherein providing a pre-selected range comprisesproviding a pre-selected range that is greater than a referencetemperature.
 13. The method of claim 11 wherein providing a pre-selectedrange comprises providing a pre-selected range that is greater than amaximum operating temperature of the light source.
 14. The method ofclaim 11 wherein maintaining the lens temperature comprises adjustingheating element current to maintain the lens temperature greater than amaximum operating temperature of the light source, thereby minimizingchanges in a front focal length of the lens caused by temperaturevariation.
 15. The method of claim 14 further comprising actuating anindicator when the lens operating temperature is greater than themaximum operating temperature of the light source.
 16. The method ofclaim 11 wherein providing a pre-selected range comprises programmingthe range into a controller.
 17. An optical system focal lengthstabilization arrangement comprising: at least one heating element; atleast one temperature sensor; a controller in electrical communicationwith the at least one heating element and the at least one temperaturesensor; a lens monitored by the at least one temperature sensor and inheatable communication with the at least one heating element; a lightsource directing light through the lens; a housing carrying the lenstherewithin and supporting the light source; the housing furthercarrying the at least one heating element and the at least onetemperature sensor; and whereby the controller monitors a lenstemperature of the lens with the at least one temperature sensor andadjusts a heat output of the heating element to maintain the lenstemperature within a pre-selected range.
 18. An optical system with afocal length stabilization arrangement comprising: a controller arrangedto receive a temperature signal an send a heat output control signal; atleast one heating element arranged to receive and respond to the heatoutput control signal from the controller; at least one temperaturesensor arranged to send the temperature signal to the controller; ahousing supporting: a lens mounted within the housing, monitored by theat least one temperature sensor, and in heatable communication with theat least one heating element; a light source directing light through thelens; the at least one heating element; and the at least one temperaturesensor; and whereby the controller monitors and adjusts the heat outputsignal in response to the temperature signal to maintain the lenstemperature within a pre-selected range.
 19. A focal length stabilizedoptical system comprising a controller, at least one heating elementresponsive to the controller, at least one temperature sensor sending atemperature signal to the controller, a lens monitored by the at leastone temperature sensor and heatable by the at least one heating element,a light source directing light through the lens, and a housing in whichthe lens is mounted and on which the light source, the at least oneheating element, and the at least one temperature sensor are mounted,whereby the controller receives the temperature signal and adjusts aheat output of the heating element to maintain the temperature within apre-selected range.